Polystyrene foams have become popular because they are relatively rigid, they are very light, and they are good insulators. As a result of these properties the foamed polystyrene container has a great proportion of the packaging, the food container and the beverage container markets, a total of billions of pounds a year of polystyrene foam in all.
There are problems with the foamed polystyrene product. The lightweight property that makes it so useful is also a major problem. The fine cells of the foam do not easily compact and the product takes large volumes in areas such as landfills and is so fluffy and light it is hard to recycle. As cups break up they tend to float and blow in a most annoying manner and can be a real source of visual pollution. Also, being a synthetic resin, there is little natural degradation of the base polymer so the foam particles stay around for a long time.
The landfill volume and the durability problems have resulted in major unfavorable publicity for the polystyrene foam product industry. While there is more plastic in many paper cups than in a foam beverage cup, the foam cup is very visible and has become a focal point of the debate on recycling. There have been bans on use of polystyrene food containers within towns due to the load placed on landfills, boycotts and picketing of users by concerned citizens and environmentalists, and substitutions of other materials. The uses and advantages of foam continue but there is a growing movement to require recycling of foam products as well as other plastic materials. A need exists to make polystyrene foam either landfill volume compatible with other types of trash or to find an effective way to recycle it.
In the case of polystyrene foam products, recycling is not easy. In one well documented case, McDonalds in Rochester, NY pledged to recycle from three outlets located in that city. The polystyrene is segregated in separate containers on the floor of the food service area by customers and the bags of foam product from those containers are removed when full and stored. At regular intervals the foam filled bags are trucked to the nearest recycler in Springfield, MA. The truck which is a standard 45 foot trailer and tractor normally would hold up to 40,000 pounds and the cost of the trip, approximately $1500 would amount to a net cost of under 4 cents per pound. The lightweight foam is not a normal material and in the bagged foam a typical load is only 1000 pounds which increases the cost to $1.50 per pound for shipping. For a material that sells as virgin resin pellets in the $0.50 to 0.60 per pound range, the shipment costs alone are nearly three times the maximum value of the recycle. There is a pressing need for volume reduction of foams to make shipping effective.
The problem with recycle does not stop at the shipment costs since a very high volume of material that weighs little must still be reprocessed. The lightweight foam must be reduced in some manner. As noted above it is a good insulator so thermal softening of the foam to compact it requires long times and during these extended low level heatings the resin may be degraded. Compaction by normal means is totally ineffective due to the multiplicity of cells that must be broken. Several methods have been developed such as the poly puff where material is compacted by shear heating against the sides of a drum as a rapidly rotating rotor mixes and stirs the material and use of crammers and enlarged feed section extruders. None of these methods are very effective in reducing foam volume and there is a need for an effective way to introduce foam materials to an extruder or other reprocessing machine.
Once in a typical extruder the normal screw would compact resin by a ratio of under 3 to 1. This compaction is due to a change in the depth of the root of the screw and in addition in some screws it would also be due to a change in the pitch of the screw. As noted before the foam can be reduced thirtyfold before the foam is completely broken down. A screw with flight depth to accomplish a 30:1 volume reduction would not have enough strength to turn due to the extreme depth needed in the root at the feed area and the very narrow outlet end would overshear the polymer and overheat or degrade it. There is a need for a foam volume reduction method to allow use of more normal plastic recycle and processing equipment. At present extra compaction steps and very expensive and specialized equipment is required to recycle polystyrene.
One aproach to the recycling is to use chemicals to reduce the foams. The basic problem is that the chemicals that are obvious are very toxic to the environment and they are often banned by SARA or similar environmental legislation. One chemical series, pinene and terpenes such as d-limonene to reduce the foam volume. This approach is interesting but it fails in some cases. There is an inconsistent activity in collapsing foams that has not previously been noticed. There is a need for such a process but that process must be rapid and effective to have commercial value.
In general all of these problems at present make the recycling of polystyrene foam non-economic and as a result it will not become a commercially viable concept until the shipment and extrusion of polystyrene foams can be reduced in cost. It will also be needed to improve the methods of converting the foams back into pellets or products for further use of the recycled polystyrene.